Two types of electromagnetic current meters were used in the DELILAH
array, Marsh-McBirney 551's (Figure 4 in the main text) and Scripps
Institution of Oceanography "open frame" sensors (Figure D1). All of the
current meters were calibrated in a constant flow tank by the Naval
Oceanographic Office in Bay St. Louis, MS. Each instrument was calibrated at
five speeds ranging from 0.0 to 1.91 m/s and four probe orientations
(± X, ±Y).
All instruments deployed during the experiment were also
postcalibrated. Because of the biological growth that developed on the open
frame current meters, they were post-calibrated with and without the growth.
Although the growth had decayed somewhat by the time the instruments were
recalibrated, the results give some indication of their performance under fouled
conditions. The calibration results, including pre- and postcalibration gains and
bias values, and computed changes between the calibrations, are listed in
Tables D1 and D2.
Pre- and postcalibrations were used to adjust the collected time series data.
In general, the precalibration was used unless the postcalibration was
significantly different. Then, either the postcalibration was used or the pre-
and postcalibrations were averaged. The decision to use only the
postcalibration was based primarily on a comparison with nearby gauges during
a time of high flow over linear bathymetry. In most cases, the current meter
offsets were determined using the calibration curve for each channel of a given
gauge and applied as a bias in volts. After plotting
the time series in engineering units, however, further offsets of the velocities
from zero were obvious for open frame channels 2711, 2731, and 2741 in the
crest subarray, and for Marsh-McBirney channels 2701 and 2901 in the
primary cross-shore subarray. These channels all measured cross-shore
currents. The offsets appeared to be systematic throughout the duration of
each time series, suggesting that a constant value applied to these data would be
a satisfactory adjustment. To correct for these offsets, current velocities taken
from these channels during a calm period (6-10 October) were correlated
against data from channels which did not appear to require further offsets. The
y-intercept of the regression curve for each channel was the offset applied in
meters per second to the current velocities. Correlation coefficient (r2) values
for these regressions ranged from 0.77 m/sec to 0.90 m/s. The greatest offset
of -0.231 m/s was applied to channel 2741. New biases were computed from
the changed offset using the following equation:
biasnew = biasorig. + (Doffset / gainorig.).(1)
The open frame electromagnetic current meters posed a particular problem
because of unanticipated biological fouling. Normally, the offset can be field
determined by comparing data collected at orientations of 0 deg and 180 deg.
However, the high wave activity during the last 2 weeks of the experiment
prevented this check from being accomplished. An analysis performed on the
current meter data from the crest and trough subarrays indicates an increase in
the gains of the open frame sensors from the beginning to end of the
experiment. This analysis is described in detail in the "Data Processing"
portion of the Stationary Instrument Data section.
A comparison calibration plot for the for sensor S385-X, where the post-calibration
differed from the pre-calibration, is shown below.
All other sensor calibration plots can also be viewed from this table.
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